1. Field of the Invention
The present invention relates to an indium phosphide (InP) metal semiconductor field-effect transistor (MESFET), and more particularly, an InP MESFET including a semiconductor surface barrier layer, in which the barrier height across the interface of a gate electrode and the semiconductor surface barrier layer is higher than the barrier height across an interface of the gate electrode and InP without the semiconductor surface barrier layer.
2. Description of the Related Art
Field effect transistors formed on InP are suitable for applications in microwave systems, millimeter wave systems, high speed digital systems, and optoelectronics. These applications of field effect transistors, and these systems, could, for example, involve integrating an InP low noise or low power amplifier with photodetectors or microwave or millimeter wave sources. Field-effect transistors formed on InP devices are desirable because InP can be lattice matched to materials such as InGaAs and InGaAsP which are used as photodetectors, and because InP has a high breakdown voltage when compared to silicon (Si) or gallium arsenide (GaAs). Furthermore, InP has a high radiation tolerance and the electron velocity in InP is higher than the electron velocity in GaAs -- two factors which make InP desirable for microwave and millimeter wave applications. Moreover, optical fiber communication has an optimum frequency in the region of the bandgap of InGaAsP.
MESFETs formed on InP, however, have produced poor results because of the low barrier height across the interface of the gate electrode and n-type InP. In particular, GaAs MESFETs having barrier heights of 0.8-0.9 eV have lower noise figures and higher gains than equivalent InP MESFETs with barrier heights of 0.45-0.55 eV, as set forth in "InP Schottky-Gate Field-Effect Transistors," by J. S. Barrera and R. J. Archer, IEEE Transactions on Electron Devices, Vol. Ed-22, No. 11, p. 1023 (1975) and "Ion-Implanted n-Channel InP Metal Semiconductor Field-Effect Transistor," by Gleason et al., Applied Physics Letters, Vol. 32, p. 578 (1978).
The low barrier height across the interface between a metal (gate electrode) and InP has resulted in the use of InP field-effect transistors having a metal-insulator-semiconductor sandwich structure (MISFET). The insulator placed between the metal gate electrode and the InP semiconductor may be Si.sub.3 N.sub.4, SiO.sub.2, Al.sub.2 O.sub.3, or oxides of InP. MISFETs, however, have several disadvantages: (1) the DC characteristics of MISFETs tend to drift because of the low quality of the insulators, which can lead to changes in the operating point of an amplifier in high frequency applications, resulting in changes in the gain; (2) irregularities at the insulator-semiconductor interface reduce the electron velocities due to scattering, thereby preventing the device from operating at the highest predicted frequencies; and (3) MISFETs are inherently less radiation hardened than MESFETs, and therefore are less suitable for military and space applications